The present invention is related to gas turbine engines, and in particular to variable vane counterbored holes for engine casings.
Gas turbine engines operate by combusting fuel in compressed air to create heated gases with increased pressure and density. The heated gases are ultimately forced through an exhaust nozzle, which is used to step up the velocity of the exiting gases and in-turn produce thrust for driving an aircraft. In turbofan engines the heated gases are used to drive a turbine for rotating a fan to produce thrust, and to drive a turbine for driving a compressor that provides the compressed air used during combustion. The compressor section of a gas turbine engine typically comprises a series of rotor blade and stator vane stages. At each stage, rotating blades push air past the stationary vanes. Each rotor/stator stage increases the pressure and density of the air. Stators convert the kinetic energy of the air into pressure, and they redirect the trajectory of the air coming off the rotors for flow into the next compressor stage.
The speed range of an aircraft powered by a gas turbine engine is directly related to the level of air pressure generated in the compressor section. For different aircraft speeds, the velocity of the airflow through the gas turbine engine varies. Thus, the incidence of the air onto rotor blades of subsequent compressor stages differs at different aircraft speeds. One way of achieving more efficient performance of the gas turbine engine over the entire speed range, especially at high speed/high pressure ranges, is to use variable stator vanes which can optimize the incidence of the airflow onto subsequent compressor stage blades.
A plurality of variable stator vanes are typically circumferentially arranged between outer and inner diameter shrouds, which are typically manufactured from steel alloys. The vanes typically include trunnion posts at their innermost and outermost diameters that extend through counterbored holes in the shrouds, respectively. Accordingly, it is desirable that the variable vanes have low-friction rotational movement within the counterbores. However, over the course of an engine lifetime, these counterbores become worn and weathered. In addition to normal vane-induced wear, operation in wet and/or salt-rich environments induces corrosion or pitting in the counterbores, which interferes with free rotation of the vane trunnions within the counterbores. In the case of severe wear or corrosion, it can be necessary to replace the entire compressor case or vane shroud in order to restore optimal free rotation to the variable vanes. This is undesirable because these parts are typically very costly due to the high-grade alloys and precision manufacturing necessary to produce these parts. Thus, there is a need for improved methods and systems for reducing or eliminating the effects of wear and corrosion on variable vane counterbored holes.